With a background of social problems such as global warming and an increase in crude oil prices, there is a rapid spread of hybrid electric vehicles (HEVs) and other vehicles targeted at a high mileage. In general, an HEV includes a main high-voltage battery for driving an engine assisting motor and an auxiliary low-voltage battery for supplying electric power to electronic devices mounted on the vehicle. The main high-voltage battery is charged when the engine rotates the motor and produces (regenerates) electric power. The generated electric power is converted by a DC-DC converter to electric power for the auxiliary low-voltage battery and supplied to the vehicle-mounted electronic devices. Thus, the main purpose of the DC-DC converter disposed between the main high-voltage battery and the auxiliary low-voltage battery is to cause a step-down operation from the main high-voltage battery to the auxiliary low-voltage battery. However, there is also a need to cause a step-up operation from the auxiliary low-voltage battery to the main high-voltage battery. For example, the engine may not be capable of being started due to a low voltage of the main high-voltage battery. In this case, if electric power can be supplied from the auxiliary low-voltage battery to the main high-voltage battery, the auxiliary low-voltage battery can compensate for the power insufficiency to start the engine through the main high-voltage battery alone. Accordingly, a bi-directional DC-DC converter having both a step-down function that serves from the high-voltage side to the low-voltage side and a step-up function that serves from the low-voltage side to the high-voltage side is demanded.
Examples of the prior art related to this type of bi-directional DC-DC converter are disclosed in, for example, Patent Documents 1 to 3.    Patent Document 1: Japanese Patent Laid-open No. 2003-111413    Patent Document 2: Japanese Patent Laid-open No. 2002-165448    Patent Document 3: Japanese Patent Laid-open No. 11 (1999)-8910